B088 EM4 CDS REV3 - WhatDoTheyKnow

Conceptual Design Statement TLF- 444-V6

Document Ref. No: TLL-B088-P954-EST-STM-00001 Revision

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Management System

Form:

Conceptual Design Statement Civils

Reference Number: TLF-444-V5.1

From Parent Document: P-308

Project Title:

Embankment Stabilisation Permanent Works, B088/EM4 Kingsbury to Wembley Park

A Conceptual Design Statement must be provided by the designer to describe the nature of the project and the proposed asset scope and to confirm that the design will be fit for purpose and compliant with applicable standards. It is intended that this Statement is discipline specific so there will be one of these forms for each asset group involved in the project scope.

SECTION A 1. Location

Embankment slope B088/EM4 is located adjacent to the southbound Jubilee Line track, from LU LCS chainages B088/JSB1070 to JSB1615. It has a total length of approximately 545m and is located between Kingsbury and Wembley Park Stations. The asset location plans are presented in Appendix A.

2. Outline of works (including definition of new assets and impact on existing assets)

London Underground - Capacity Optimisation Programme (LU COP) Earth Structures team have been commissioned to carry out conceptual design for embankment B088/EM4. The embankment was re-assessed in 2014 by LU COP [1] and was assigned an ACA classification of 76% E2 (Poor), 14% D (Poor) and 10% A (Serviceable). Important Note: Both visual evidence and geotechnical monitoring has indicated slope instability between chainages B088/JSB1330 to JSB1370. The recorded movements have breached the ‘Amber’ trigger level defined in the Earth Structure Assessment – Emergency Preparedness Plan [8]. As a result, an engineering review panel meeting has been convened and possible emergency stabilising measures explored. These potential emergency works do not form part of this CDS and will be addressed in separate documentation. This CDS considers various remedial options to upgrade the whole of the embankment to an ACA classification of ‘100% A’, in accordance with Standards LU 1-054 and LU G-031. The effect the upgrade will have on existing structures/assets will be considered and assessed, so as to have no detrimental effect on their stability. This CDS covers the permanent works only, with the associated temporary works to be addressed in a separate temporary works design statement (TWDS) TLF-716.

3. Existing Site Conditions and Access

See Section B.6. (No F3333 Works Notification and Risk Assessment has been undertaken at present.)

DRACCT Applicability No Document No. TLL-B088-P954-ASS-APL-00001



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4. Interface/Impacts on other Asset Areas See Sections B.6. and B.7.

5. Third Party Approvals Required - Approval required from the Environment Agency (EA) and Thames Water (TW), regarding third party

asset protection of existing structure W503 (sheet piled wall with concrete capping beam). - Approval required from EA regarding flood defence consent and drainage discharge consent.

6. Special Site Requirements At present, there are no special site requirements.

7. Special Requirements for Procurement

None envisaged. 8. Maintenance Strategy

See Section B.14. 9. Principal Standards Group Used

- All law, regulator’s guidance, BS/EN standards, and LUL category one standards as identified in Project Assurance Plan (PAP), Document No. TLL-L001-SMEP-ASS-PLN-00002.

10. Concessions expected to be sought None envisaged for the permanent works.

11. Noted exceptions/omissions

None. 12. Attachments

No Scope of Works report or historical reports have been attached.



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SECTION B Other Specific Considerations CIVIL

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Item Description

1.0 Title of scheme: Embankment Stabilisation Works, B088/EM4 Kingsbury to Wembley Park

(PER = P954)

2.0 Identification of structure:

Identification:

Embankment B088/EM4, Kingsbury to Wembley Park.

Location:

Embankment slope B088/EM4 is located adjacent to the southbound Jubilee Line track from LU LCS chainages B088/JSB1070 to JSB1615. It has a total length of approximately 545m and is located between Kingsbury and Wembley Park Stations. The asset location plans are presented in Appendix A.

Description:

The slope form is variable along the embankment length and reaches a maximum height of approximately 7m-8m at chainage JSB1615, before reducing in height to the north until it reaches natural ground level at chainage JSB1070. The embankment slope has an average angle of 20° to 25° and is generally split into two slopes by a mid-slope terrace with an approximate slope angle of 2° to 10°. The Wealdstone Brook runs through a bridged culvert (S3) beneath the embankment at chainage JSB1320 and continues to run through an open culvert parallel to the toe of the embankment up to JSB1615. North of the culvert, the toe of the embankment slope is bounded by residential gardens on Uxendon Hill. The southern end of the embankment terminates at bridge S2.

3.0 Functional requirements (reason for works):

Where practicable, to increase the current ACA to ‘100% A’ in accordance with Standards LU 1-054 and LU G-031.

To reduce future deformation of the earth structure and minimise the risk of damage to the existing structures, track or services sited on or adjacent to the embankment.

The design should endeavour to maximise ease of access for operational and maintenance purposes. This shall include the provision of a minimum 1.00m width access walkway located at the crest of the embankment, along with stepped access leading from the toe to the crest.

To reduce the susceptibility of ponding at the toe of the embankment.

To provide a landscaping scheme to replace vegetation removed as part of the enabling works.

4.0 Name of sponsor (TLL, Contractor, Outside Party):

London Underground - Capacity Optimisation Programme (LU COP).

15 Westferry Circus, London, E14 4HD.



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5.0 Proposed dates of project start and completion:

Design - TBC

Construction - TBC

6.0

Brief description of existing conditions:

Existing Structures:

A summary of the existing structures on embankment B088/EM4 are summarised in Table 6.1 below. A detailed description of the structures, along with ‘As-Built’ and historical drawings are detailed in the LU COP Earth Structure Assessment (ESA) Report (2014) [1], and in an Operation and Maintenance Manual for structures W502/W503 [9]. Structural assessments and inspection reports have been obtained for the existing structures [10,11,12], with the exception of W502, W503 and W670.

Table 6.1 Summary of Existing Structures Identified on Embankment B088/EM4

Earth Structure

Asset

Structure Asset Number

Description

B088/EM4

W502 Reinforced concrete open culvert [9].

W503 Steel sheet piled wall with concrete capping beam [9].

W518 Reinforced concrete wall [10].

S3 Reinforced concrete bridged culvert [11].

W670 Piled reinforced concrete retaining structure.

S2 Single span iron girder bridge, supported by brick abutments and wing walls [12].

As part of the detailed design, a full condition survey of all existing structures will be required, along with an assessment of the potential impacts of any proposed permanent or temporary works. Structural assessments inclusive of AIP’s, of W502, W503 and W670 will also be required. This will allow consideration of the structures functionality and performance, along with the design of structure protection during any physical works.

As discussed in the LU COP ESA Report (2014) [1], structure W670 is currently under ‘Special Inspection’ due to a ‘severe’ defect in its alignment. Monitoring of the structure shows that the structure continues to move horizontally away from the track. This movement and its effect on the structure/embankment stability will need to be investigated as part of the detailed design.

Lineside Services:

A single 2nd generation cable run is mounted on steel posts that range from approximately 1.0m to 1.2m in height. The cable run is located on the slope crest and runs the length of the embankment. At present, it is in a serviceable condition.



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Utility and Third Party Services:

No utility or third party services search has been undertaken at present. Prior to undertaking the detailed design, a full search should be commissioned, to identify any utility and third party services that may affect the proposed design and subsequent physical works.

Drainage:

Between chainage JSB1070 to JSB1320, the remnant of a drainage ditch runs parallel with the toe of the embankment. The ditch is almost completely in filled with soil and detritus. Significant flooding at the toe of the embankment has also been noted in this area during wet winter months. In places, it has been recorded as up to 300mm deep and flooding the gardens of the residential properties that bound the embankment toe. A slow downslope flow towards the culvert at chainage JSB1320 has been observed, where it accumulates behind the bridged culvert structure and eventually discharges into the Wealdstone Brook.

At approximate chainage JSB1100, a single catchpit is located at the toe of the embankment, with a single drainage carrier pipe infall and outfall. The pipe runs parallel with the toe of the embankment and is believed to connect to track drainage further north of the embankment. The outfall pipe terminates within a poorly defined ditch, which contributes to the flooding of the toe further downslope.

Between chainages JSB1320 to JSB1615, no visual evidence of drainage has been observed.

Prior to undertaking the detailed design, a full drainage survey should be undertaken to investigate the condition, line and level, alignment and extent of any existing catch pit and drainage pipes.

Vegetation and Wildlife:

The embankment is densely vegetated with a mixture of mature deciduous and evergreen trees and shrubs, with dense brambles at the crest. Various tree species have been noted including high water demand species such as sycamore, false acacia, willow and English oak. Hydrophilic plants were observed at the toe of the embankment, where evidence of ponding has been observed.

Wildlife activity in the form of burrowing was observed across the embankment, particularly in the ash at the crest. No further evidence of wildlife activity has been observed on the embankment.

Prior to undertaking the detailed design, a full environmental survey should be undertaken. This may identify any environmental constraints that will need to be considered in the detailed design, or remedial actions that will need to be taken prior to undertaking any physical work.



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Contamination:

No recent contamination testing has been undertaken on the embankment. Prior to undertaking the detailed design, a preliminary contamination survey should be undertaken to gain an understanding of the potential pathways/receptors and the basic preliminary costs to remove material from site.

The risk to workers and requirements for additional detailed contamination testing should be addressed at the detailed design stage, in the designers Risk Assessment.

Prior to undertaking the detailed design, an ‘F3333 Works Notification and Asbestos Material Risk Assessment’, should be commissioned for the embankment.

Access:

The only current vehicular and pedestrian access to the embankment is through a double gate off The Avenue, at approximate chainage JSB1615. Access along the embankment is significantly restricted due to the dense vegetation, proximity of the culvert and lack of space between the toe and LU boundary fenceline. More detailed discussion on the access options and restrictions is detailed in the LU COP Feasibility Report (FR) (2015) [2] and will be considered during the detailed design stage.

Track Quality:

Track Recording Vehicle (TRV) data for the period January 2010 to December 2014 has been obtained from the Track Quality Engineer. This data has been examined and shows that that at present, the track condition is ‘satisfactory’ for the entire embankment. However, it does show that within the past four years, track performance has at times deteriorated, with maintenance intervention required to keep it is a ‘satisfactory’ condition. A more complete assessment of the track quality is detailed in the LU COP ESA Report (2014) [1].

Ground Investigations:

Soil Mechanics Limited (1994) - A ground investigation was undertaken by Soil Mechanics Limited (SML) on two separate occasions, as reported by SML in February 1994 [3] and February 1995 [4]. The results of these ground investigations are summarised in a subsequent Howard Humphries GIR (1995) [5]. The ground investigation comprised:

Trial Pits, Boreholes and Dynamic Probing, along with in-situ testing and soil sampling.

Laboratory geotechnical testing, including geotechnical index, effective stress, soil strength and soil chemistry testing.

Installation of geotechnical monitoring instrumentation in the form of standpipe and vibrating wire piezometers.



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LU COP (2013-2014) - A ground investigation was undertaken by Endeavour Drilling Limited (EDL), under the supervision of LU COP, in July 2013 and July/October 2014. Six transects were investigated, spaced across the embankment and centred on areas where a walkover survey (2014) indicated potential slope instability. The scope was to determine the stratigraphy and geotechnical properties of the embankment and underlying strata. In addition at each transect, geotechnical instrumentation was installed to monitor piezometric levels and slope movements within the embankment over a specified period. The ground investigation was undertaken in accordance with LU Standard 1-050 and BS EN 1997-2:2007 Eurocode 7: Geotechnical Design, Ground Investigation and Testing. The results of this ground investigation are presented in the LU COP GIR (2014) [6] and summarised as follows:

Twelve Modular Window Sample (MWS) boreholes, along with in-situ testing and soil sampling.

Laboratory geotechnical testing, including geotechnical index, effective stress shear strength and soil chemistry testing. All laboratory testing was undertaken by the UKAS accredited Professional Soils Laboratory Limited (PSL). The geotechnical tests were carried out in accordance with standard procedures given in BS 1377:1990: Methods of testing for Soils for Civil Engineering Purposes.

Installation of geotechnical monitoring instrumentation in the form of four duel standpipe piezometers (DSP) and seven inclinometers.

Survey of in-situ borehole location by LU COP survey team.

Survey:

A detailed topographical survey of the embankment was undertaken in 2013 by the LU COP survey team. The survey was undertaken to allow short and long-term slope stability assessment of the earth structure and the subsequent design of enabling and permanent remedial works. The survey was completed in accordance with LU Engineering Standard 1-026 A2 - Topographical Surveys and Mapping.



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Local Geology and Ground Model:

The 1:50,000 British Geological Survey Sheet 256 [7], North London, published in 1994, indicates that the London Clay Formation underlies the site, possibly with localised deposits of Alluvium. Ground investigations carried out by Soil Mechanics (1994/1995) [2, 3] found the embankment to have a clay core, with a varying thickness of ash at the crest and shoulders. It was found to be constructed on thin layers of Relic Topsoil and Alluvium, overlying the London Clay Formation. The ground investigation completed by LU COP in July 2013 to July/October 2014 and summarised in the LU COP GIR (2014) [6], encountered the same strata sequence as detailed above, however two distinct types of embankment fill were identified. EFC A was interpreted as being sourced from Weathered London Clay materials and EFC B sourced from Alluvium materials. A cross section of the typical ground model is presented Figure 6.1 below.

Figure 6.1 Typical Ground Model

Based on the current ground investigation information, the Ash appears to vary significantly in thickness across the embankment. Slope stability analysis undertaken in the LU COP ESA (2014) [1], found that the depth of Ash has a significant effect on the embankment stability. Prior to undertaking the detailed design, further ground investigation should be undertaken to define the thickness of the Ash at each survey transect (approx. 25m spacing), this will help to optimise the detailed design.

It is also considered appropriate to refine the ground model and soil parameters, by drilling a further four boreholes, with full geotechnical in-situ and laboratory testing. Two should be located on the upper slope at approximate chainages JSB1550 and JSB1275, along with two located at the toe behind the sheet piled wall (W503), at approximate chainages JSB1550 and JSB1360.

30kN/M²

Ballast

Ash

EFC A

EFC B

Alluvium

Weathered London Clay

Weakened Basal Layer

Cable Run SB Jubilee Line track

Fenceline

PiezometricLevel

London Clay



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Hazard and Risk Classification:

The LU Earth Structures Design Guide provides guidance and flow charts, to determine the hazard potential and risk classification of progressive deep-seated failure as the dominant stability deformation mechanism on the embankment. Table 6.2 below, summarises the classification for Embankment B088/EM4.

Table 6.2 Summary of Hazard/Risk Classification for Embankment B088/EM4

Risk Classification

High Hazard High Risk

High Hazard Intermediate Risk

High Hazard Low Risk

Chainage JSB1300 - JSB1600 JSB1100 - JSB1300 JSB1070 - JSB1100

Clay Fill CF >44% / LL >60% CF >44% / LL >60% CF >44% / LL >60%

Clay Core Height

>4.00m >2.00m - <4.00m <2.00m

Underdrained No No No

As a result of these classifications, recommendations in the LU Earth Structures Design Guide have been used where applicable, along with laboratory testing results to determine the soil parameters and approach to be used in long-term slope stability assessment of the embankment.

Groundwater:

Monitoring of DSP’s installed by EDL in July 2013, has been undertaken for over one year. As discussed in the LU COP ESA (2014) [1], groundwater levels within the embankment rose sharply during the wet winter months, with a maximum-recorded level of 0.70m below the Ash/EFC interface (138.45m AOD) at transect JSB1360. The winter of 2013-2014 has been confirmed as one of the wettest on record, therefore, whilst the monitoring has only been undertaken for one year, it can be assumed to reflect worst-case in-situ groundwater levels. There is also evidence of poor drainage on the embankment, with ponding/flooding recorded at the toe.

Soil Chemistry:

The LU COP GIR (2014) [6], reviews the soil chemistry test results from the EDL ground investigation (2014). Characteristic values indicate that ground conditions potentially aggressive to concrete are present in the embankment. BRE soil and water test suites were undertaken on a selection of representative soil and water samples. The results of which determine a Design Sulphate Class of DS-3, with an Aggressive Chemical Environment for Concrete Classification of AC-3 for mobile groundwater conditions, in accordance with BRE Special Digest 1.



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6.1

Proposed additional surveys or investigations:

Additional ground investigation to include:

a) An Ash thickness investigation, centred at each survey transect at the crest of the embankment (approx. 25m spacing).

b) Two boreholes located on the upper slope at approximate chainages JSB1550 and JSB1275.

c) Two boreholes located at the toe of the embankment behind the sheet piled wall (W503), at approximate chainages JSB1550 and JSB1360.

Preliminary soil contamination survey, to include Waste Acceptance Criteria (WAC) testing. Where removal of soil from site is required, the chemical properties of the soil will need to be determined by lab testing, so that it can be classified in accordance with the Waste Acceptance Criteria Procedure.

A F3333 Works Notification and Risk Assessment should be commissioned. The results of this will be used during detailed design and provided to the contractor when carrying out any physical works.

An environmental assessment should be undertaken prior to the detailed design of remedial works on the embankment. This should include, but not be limited to, a review of vegetation, small mammals, nesting birds, reptiles and bats.

Structural assessment of existing structures W502, W503 and W670.

Condition survey of all existing structures and collation of all ‘As-Built’ and historical drawings and reports.

A full utility and third party services survey should be commissioned to identify any utility or services that may affect the proposed detailed design and subsequent physical works.

Third Party permission should be obtained for the site access and compound, as discussed in the LU COP FR (2015) [2]. Any restrictions will need to be considered during the detailed design.

As recommended in the LU COP FR (2015) [2], an Unexploded Ordnance (UXO) survey should be commissioned to establish the risk of unexploded ordnance on the site. The results of this will be considered during detailed design and provided to the contractor.

Flood defence and drainage discharge consents to be obtained from EA. Any restrictions will need to be considered during the detailed design.

Approval required from the EA/TW, regarding third party asset protection of existing structures W502 and W503.

Drainage survey to investigate the condition, line and level, alignment and extent of any existing catch pit and drainage pipes.



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7.0 Description of proposed works and method of construction with reasons for choosing:

The LU COP FR (2015) [2] discusses in detail the proposed remedial options considered for the embankment. Following assessment of these options, the following preferred remedial solution was considered appropriate for the permanent works, with the reason for choosing summarised in Table 6.3.

Discrete Bored Piles

Along with:

Minor Granular Replacement/Granular Capping Layer

Toe Drainage

Table 6.3 Summary of Preferred Remedial Option / Reason for Choosing

Preferred Remedial Option / Reason For Choosing

Discrete Bored Piles

Provide long-term stability against the deep-seated instability, without drastically changing the existing slope profile.

Can be constructed in continuous rows where required, depending on the embankment height and width.

Designed such that they terminate below the final slope profile and therefore are not visible once the works are complete.

Require no maintenance post construction and adhere to the requirement for a 120-year design life, as defined in LU Standard 1-054.

No interface or impact with any existing structures or third party assets on the embankment.

Recently used to successfully stabilise comparable embankments at Canons Park (B084/EM1 and EM4), therefore a recent case study can be drawn upon to review the lessons learnt and optimise the design.

Minor Granular Replacement/Granular Capping Layer

The addition of the minor granular replacement/granular capping layer will help to stabilise the slope against shallow instability, that could occur as a result of the removal of vegetation from the slope surface.

It will also provide a capillary break between the surface and the clay in the embankment core. This should help to reduce seasonal deformations in the form of shrink swell cycles of the high plasticity clay, contributing to serviceability instability, as discussed in Section 2.8.3.

Toe Drainage

Toe drainage will reduce the susceptibility of ponding at the toe of the embankment and capture any surface runoff from the relatively impermeable compacted granular capping layer.

A conceptual drawing of the preferred remedial option is presented in Appendix B. Precise details of the design and work extent are to be determined during the detailed design following further interrogation of the topographical survey, ground conditions, slope stability assessments and drainage survey information.



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Discrete Bored Piles:

It is proposed to stabilise the slope against potential deep-seated failures by installing a series of discrete slope reinforcement piles, parallel to the length of the embankment slope. The piles will act to reinforce and strengthen the ground around them. The piles will be constructed below the permanent design slope regrade profile, therefore will not be visible following completion of the works. The piles are designed to act as discrete slope reinforcing elements, therefore no capping beam is required. They will be constructed from a sufficiently wide and suitably designed temporary piling platform, using a rotary auger piling rig. There may be a requirement to construct more than one row of discrete piles, this will be addressed during the detailed design. The pile alignment will need to be designed such that they do not clash with the extent of the existing historic remedial works, installed in 1996, such as lime piles and mix in place grout logs.

Slope Regrade/Granular Capping Layer:

Slope regrade is proposed along the length of the embankment, to provide a granular capping layer, shoulder support and a 1.00m wide crest walkway. This will stabilise the slope against potential shallow failures and support the crest without the requirement for a pile cap/retaining structure. HA Class 1A granular fill material shall be used, with an in-situ strength of no lower than ɸ’ = 40°. The fill will be benched into the existing embankment fill material and compacted in layers. The slope regrade will start at the toe and initially be over-built to form the temporary platforms from which the discrete piles will be constructed. Once piling is complete, the temporary platforms will be regraded to the permanent slope profile, to be determined during the detailed design.

Toe Drainage:

In line with LU Standard S1052-A4, it is proposed to construct a filter drain along the toe of the embankment slope, to collect any rainfall and groundwater discharge from the slope. A perforated pipe will be installed at the bottom of the filter drain and will be sloped towards the outlet to provide adequate hydraulic capacity. The proposed outlet will discharge into the Wealdstone Brook culvert, which runs parallel to the toe of the embankment between chainages JNB1320-1615. A geotextile will be placed around and over the top of the drain to prevent the infiltration of fine particles and consequent reduction in drain functionality. Catchpits will be installed at 30m intervals to enable inspection and maintenance of the carrier pipes.

Temporary Works:

The TW will be intrinsically linked to the detailed design of the permanent works; therefore, this CDS only covers the permanent works, with the TW addressed in a TWDS TLF-716. The TW are envisaged to include:

Temporary piling platforms, access roads / ramps.

Lineside services protection.

Temporary post and ply retaining walls.

Temporary ramp over overbridge S2 northeast wing wall / bridged culvert S3.

Excavations / shoring.

Third Party asset protection.

Site Compound.



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7.1 Special Finishes or Features:

A 100mm thick layer of topsoil will be placed on top of the regraded slope, with a suitable landscaping scheme developed.

Upgrade/reinstatement of compliant LU boundary fenceline.

Installation of a handrail/barrier on existing wing wall and culvert structures.

Installation of a 1.00m wide crest walkway, by placing a 150mm layer of Type 1 material behind the lineside services, to provide a surface for walking. A Terram root guard (or similar system) will be installed below the Type 1 material to prevent vegetation growth.

Installation of stepped access from the toe to the crest of the embankment, at approximate chainage JSB1615, along with a suitable handrail.

Installation of signage identifying the location of the buried discrete piles, along with their construction and spacing details.

7.2 Clearances from other structures:

Following the condition survey and assessment of the impact that any temporary and permanent works will have on existing structures detailed in Section B.6, sufficient clearance will be specified in the detailed design.

Along the entire length of the embankment, the remedial works will be constructed downslope of the existing lineside services and will not encroach beyond the lineside services.



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8.0 Brief description of other structural forms considered and reasons for their rejection:

The LU COP FR (2015) [2] discusses in detail the proposed remedial options considered for the embankment. Following assessment of these options, the following remedial solutions were discounted for the permanent works:

Table 6.4 Summary of Discounted Remedial Options Considered

Remedial Option Reasons For Rejection

Granular Replacement

This remedial option requires considerable excavation at the toe of the embankment. This is considered undesirable, owing to the low embankment fill strength and records of historic and current slope instability on the embankment. In addition, this option is considered impractical owing the very large amounts of material required to be removed and replaced with engineering fill.

Slope Regrade and Toe Berm

A lack of space exists between the toe of the embankment and sheet piled wall W503. This makes regrading to a slacker slope angle impractical.

Constructing a toe berm that is sufficient to stabilise deep-seated slope instability could risk overloading W503. In addition, it requires the importing of very large amounts of engineering fill material. For both these reasons, this option is considered impractical.

Gabion Toe Wall and Toe Berm

Constructing a gabion wall and toe berm at the toe of the embankment could risk overloading W503. Gabion wall foundations may require deep excavation at the toe of the embankment, possibly requiring significant temporary works to ensure embankment stability. Finally, it requires the importing of very large amounts of engineering fill material. For all these reasons, this option is considered impractical.

Soil Nails

This remedial option has been rejected, as it does not provide the 120-year design life required as part of LU Standard 1-054. It is considered undesirable owing to its questionable application to high plasticity clay slopes and the significant and complex maintenance requirements post construction.

Electrolysis

Whilst simple to construct and very nonintrusive, it is an unproven technique with little evidence to justify it meeting the 120-year design life required as part of LU Standard 1-054. Increased maintenance requirements post construction could also make it impractical.

Sheet Piles

The expected depth of embedment for the sheet piled wall would require significant temporary works and very large plant to install. This is considered impractical owing to the embankment space constraints and issues with very tall plant next to an operational railway.



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9.0 Design Criteria: The design criteria are based on LU Standard 1-054 A4, BS EN 1997-1 and the design methodology outlined in the LU Earth Structures Design Guide, which incorporates a progressive failure theory for deep-seated failure. The methodology is based on a hazard and risk classification for the earthwork and the geotechnical parameters depending on the classification properties of the soil materials, as summarised in the LU COP GIR (2014) [6]. Specific criteria for the proposed remedial works are:

To ensure that the FoS is at least 1.0 against deep-seated failures of the earthwork slope, when using the LU Earth Structures Design Guide and BS EN 1997-1, applying a partial factor of 1.1 on all characteristic soil parameters and a partial factor of 1.3 on the rail loading.

To ensure that the FoS is at least 1.15 against shallow failures of the earthwork slope, when using the LU Earth Structures Design Guide and BS EN 1997-1, applying unfactored characteristic soil parameters and a partial factor of 1.3 on the rail loading.

All proposed permanent earth structures will be required to have a design life of 120 years. Both long term and short term conditions will be considered in the design in order to take account the most onerous requirements, which can be reasonably foreseen during both the construction and service life of the structure.

9.1 Live loads including partial safety factors where appropriate:

The design considers the ultimate and serviceability limit state of the structure and adjacent structures under long-term equilibrium conditions. The design of remedial works takes into account all permanent and transient loads. Temporary loads from construction plant etc. will be covered in a separate TW design statement (TLF-716).

RL railway live loading of 30kN/m2 from sleeper end to sleeper end will be applied in accordance with LU Standards S1-051 A3 and 1-054 A4. A partial factor of 1.3 will be applied to this loading.

9.2 Soil parameters (with reference to soil reports):

The LU COP GIR (2014) [6] reviewed the results from the recent ground investigation undertaken by Endeavour Drilling (2013-2014), in-situ testing and laboratory testing, to enable classification of the materials and the selection of suitable soil parameters for short and long-term slope stability assessment.

The approach was to assume parameters recommended in LU Standard 1-054 A4, along with derivations in the LU Earth Structures Design Guide, unless sufficient evidence was available that other parameters would be more appropriate. A partial factor of 1.1 will be applied to all soil layers, as defined in the LU Earth Structures Design Guide.

A summary of the soil parameters used in the long-term slope stability analysis is presented in Table 6.5.



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Table 6.5 Soil Parameters for Short and Long-Term Slope Stability Analysis of B088/EM4

Material Type

Strength State

c’ (kPa

)

’ (deg)

c’r (kPa)

’r (deg)

(kN/m³)

Cu (kPa)* Source

Railway Ballast

Critical State 0 40 - - 18 - LU

Standard 1-054

Ash Critical State 0 35 - - 11 - LU

Standard 1-054

EFG Critical State 0 35 - - 18 - LU

Standard 1-054

EFC A Cautious Peak /

Residual 2.5 17 - - 19 44

LU ES Design

Guide / GIR Laboratory Testing [6]

EFC B Cautious

Peak 2.5 19 - - 19 27

LU ES Design


Weakened Basal Layer

Residual - - 1

’r) 14.5

19 -

LU ES Design


’int) 17

(’cv) 19

Alluvium Cautious

Peak 2.5 19 - - 19 -

LU ES Design


Weathered London

Clay

Cautious Peak

2 21 - - 19 54 LU

Standard 1-054

London Clay

Cautious Peak

2 21 - - 19 - LU

Standard 1-054

* For undrained short-term analysis only.

These parameters will be reviewed following the additional ground investigation discussed in Section B.6.1 and finalised during the detailed design. In addition, parameters for imported granular fill material, used as part of the adopted remedial option will also be defined during the detailed design.



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Item No

Item Description

9.3 Groundwater Condition:

For deep-seated instability, the LU COP ESA Report (2014) [1] defined a piezometric line (zero pressure line), which was applied 1.00m below the Ash/EFC interface and at the surface of the toe of the embankment, with pore water pressures assumed to be 100% hydrostatic. This approximately reflected the worst-case monitored groundwater level to date, evidence of poor drainage and ponding at the toe of the embankment.

For shallow-seated instability, the LU COP ESA Report (2014) [1] defined a piezometric line at the Ash/EFC interface, with pore water pressures assumed to be 100% hydrostatic, as recommended by the LU Earth Structures Design Guide.

The groundwater condition adopted for the ESA [1] was based on a year of monitoring data from the DSP’s. When undertaking the detailed design, any subsequent monitoring data should be reviewed along with guidance in the LU Earth Structures Design Guide and LU Standard 1-054 A4, prior to adopting a suitable groundwater profile.

10.0 Proposed method of structural analysis (including stability at all stages):

Slope Stability Analysis:

The assumptions and principles adopted for geotechnical analysis of slope stability shall be in accordance with LU Standard 1-054 A4 and the LU Earth Structures Design Guide. Analysis of the embankment will be completed using the slope stability program Slope/W (GEO-SLOPE International Ltd., July 2012, Version 8.0.7.6129), which incorporates a search routine for identifying the critical surface with the lowest FoS against instability. There are several options within the program for the method of limit equilibrium slope stability analysis, including Morgenstern and Price, Bishop and Janbu’s method. The Morgenstern and Price ‘rigorous’ method will be used in the analyses at the detailed design stage.

For the critical analyses, the remediated slope is designed to achieve the required factor of safety (FoS) of 1.0 against deep-seated failures affecting the track and lineside services.

For the critical analyses, the remediated earthwork is designed to achieve the required FoS of 1.15 against shallow slips (i.e. only passing through sloping part of embankment).

Water pressures within the slope can be defined using a variety of options available within the program, including a grid of user defined pore pressure points, phreatic surface(s) or Ru/B-bar values. The detailed design will adopt a phreatic surface with B-bar values where required, as discussed in Section B.9.3. This will be used for the stability analysis of the existing condition and the design of the remedial works, in accordance with the requirements of the LU Earth Structures Design Guide and LU Standard 1-054 A4.

Discrete Piles:

The main assumptions and principles adopted for the geotechnical and soil-structural interaction analyses for the bored pile retaining structures shall be in accordance with LU Standard 1-054 A4 and summarised below:

Analysis of the stability of the embankment using the slope stability program Slope/W (GEO-SLOPE International Ltd., July 2012, Version 8.0.7.6129). To ensure appropriate set of soil and groundwater parameters (representative of embankment conditions) are adopted, critical slopes will be back analysed. A search is carried out to identify the geometry of a critical slip surface with the minimum FoS.



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Item No

Item Description

For the critical analyses, the out of balance force, Fx, which is required to achieve a FOS of 1.0 is calculated (where partial factor on unfavourable actions are equal to 1.3 and all other partial factors are equal to 1.1).

The force, Fx, is then input into an LU COP designed spreadsheet. The spreadsheet calculation is consistent with the requirements of EC7 BS EN 1997-1 (2004) Geotechnical Design, in that the required embedment length of the piles is derived either from factored soil strengths and unfactored loads (Case C), or factored loads and unfactored soil strengths (Case B), required to achieve a factor of safety of 1.00. For simplicity, porewater pressures are unfactored. In this instance because there is no head difference across the pile alignment, the impact on the design is believed to be negligible.

The structural requirements (i.e. reinforcement, concrete strength) are derived from factored loads and unfactored soil strengths. The dimensions of the piles are derived by checking structural capacities in accordance with EC3 BS EN 1993-1-5 2007 Steel Design and EC2 BS EN 1192-1-1 2004 Design of Concrete Structures.

The external forces acting on the vertical piles above the critical slip surface are represented by Fx values. The forces acting on the vertical piles below the critical slip surface are represented by active and passive pressure profiles. Maximum bending moment acting on the vertical piles is calculated from the above limit equilibrium calculations. The vertical pile lengths are calculated to ensure sufficient vertical and horizontal capacity. Slope stability analysis is then carried out to confirm the required depth of the piles to give a FoS of 1.0.

An analysis of the upslope and downslope side of the pile ‘shear key’ is undertaken using slope stability analysis.

11.0 Standards and codes of practice to be used in design, including temporary works (only identify additional to those listed in Programme Assurance Plan):

LUL Documents

S1-538-A9 Assurance

G-031-A1 Asset Condition Assessment and Certification

S1-165-A2 Landscaping and Vegetation

S1-021-A1 Working near Structures and Mains Services

S1-114-A2 Safe Systems of Work on or near Electrical Equipment

S1-083-A2 Passive Fire Protection Systems

S1-085-A3 Fire Safety Performance of Materials

S1-156 A8 Gauging and Clearances

S1-159 A3 Track Dimensions and Tolerances

S1-050-A7 Civil Engineering – Common Requirements

S1-051-A3 Civil Engineering – Bridge Structures

S1-057-A9 Civil Engineering – Miscellaneous Assets

S1-054-A4 Civil Engineering – Earth Structures



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Item No

Item Description

S1-052-A4 Civil Engineering Gravity Drainage Systems

T0007-A2 Material and Workmanship

G0054A-A2 Civil Engineering – Earth Structures

G0054B-A1 Earth Structures – Guide for Slope Stability Analysis

Tube Lines Documents

TLL-ENG-CIV-2-092 Temporary Works – Planning and Execution

P-308-A14 Assurance of New and Altered Assets

European / British Standards

BS 6031 (2009) Code of Practice for Earthworks

BS 8002 (1994) Code of Practice for Earth Retaining Structures

BS 8004 (1986) Code of Practice for Foundations

EC7 BS EN 1997-1 (2004) Geotechnical Design: Part 1 General Rules

EC7 BS EN 1997-2 (2007) Geotechnical Design: Ground Investigation and Testing

BS1377 (1990) Methods of test for soils for civil engineering purposes, Parts 1-9

BS EN 1990:2002+A1:2005 Basis of Structural Design

EC1 BS EN 1991-1-1 (2002) Actions on Structures: Part 1-1 General

EC2 BS EN 1992-1-1:2004 Concrete Design

EC3 BS EN 1993-1-5 Steel Design

EC4 BS EN 1994-1-1 Design of composite steel and concrete structures

BS 5837 (2012) Trees in relation to design, demolition and construction – Recommendations

BRE Special Digest 1: Concrete in aggressive ground, 2001;

Highways Agency – Specification for Highway Works

National House Building Council, Building Near Trees, 2003

Specification for Piling and Embedded Retaining Walls, The Institution of Civil Engineers, Thomas Telford, 1996

British Steel Piling Handbook, January 2005, Arcelor Group

12.0 Safety considerations:

The requirements of the Construction (Design and Management) Regulations 2007 will be complied with during the design process. The works will be designed such that they can be constructed in accordance with the Contract Quality, Environmental, Safety and Health Conditions menu (QUENSH) and any relevant safety legislation. A Designer Risk Assessment will be provided at the detailed design stage.



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Item No

Item Description

It is assumed that the remedial works will be undertaken during Traffic Hours, behind the protected cable runs / a suitable physical barrier.

All cable runs, existing structures and third party assets will be protected during the remedial works.

It is envisaged that the proposed remedial works will not have any adverse impact on existing structures (e.g. lineside services, foundations) and track.

The existing embankment monitoring scheme [8] will remain in place and be upgraded to cover any proposed physical works. This scheme will remain in place under the current site specific EPP [8], until it has been demonstrated that overall stability has been achieved and any physical works have been implemented as per the detailed design.

13.0 Track clearance approval requirements:

Along the entire length of the embankment, the remedial works will be constructed downslope of the existing lineside services and will not encroach beyond the lineside services, with no permanent structures located within 3m of the southbound Jubilee Line track.

Any works less than 3m away from the track will require clearance from the relevant Track Engineer, in accordance with LU Engineering Standard S1-156-A8 Gauging and Clearances. A safe system of work must be established prior to undertaking any works adjacent to the track.

The proposed stabilisation works are envisaged not to have any detrimental effect on the adjacent Jubilee Line tracks.

14.0 Maintenance, inspection frequencies and handover arrangements to other users:

The proposed works are to be designed for a design life of 120 years with minimal maintenance requirements. Following completion of the works, the contractor shall provide an operating maintenance manual and ‘As-Built’ drawings.

Appropriate maintenance during the design life shall include:

a) Periodic inspections of the earth structure.

b) Maintenance and upkeep of the embankment toe drainage, along with installation of identification plates on newly installed catchpits.

c) Vegetation and wildlife management.

15.0 Design check category specified in design brief (category 1, 2 or 3):

See Section B.16. below.

16.0 Proposals for independent category 2 or 3 design checks (where applicable) as defined in standard 1-538:

The Category 2 check will be carried out by an independent team within LU COP and will be the responsibility of an experienced Chartered Engineer. The assets are critical to the safety and operational performance of the railway and require at least a Category 2 check in accordance with LUL Standard S1-538-A9 Assurance.



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Item No

Item Description

17.0 Reference to supporting documentation–drawings, sketches, calculations, TLL structural assessments, site investigation reports, designer risk assessments etc.:

References:

1) London Underground – Capacity Optimisation Programme, Earth Structures Assessment Programme, Earth Structures Assessment Report, Kingsbury to Wembley Park, B088/EM4, Document No: TLL-B088-P855-CIV-RPT-00006, (October 2014);

2) London Underground – Capacity Optimisation Programme, Earth Structures Remediation Programme, Feasibility Report, Kingsbury to Wembley Park, B088/EM4, Document No: TLL-B088-P954-EST-RPT-00001, (January 2015);

3) Soil Mechanics, Ground Investigation of Eight Embankments, Site D Kingsbury to Wembley Park, Contract No. CEPU/GI/102 67/BM/U, Part 1 Text Field Records and Laboratory Testing, Report No. 7848/D1, Rev 1 (February 1993);

4) Soil Mechanics, Earth Structures 1994/1995, Ground Investigation, Site B/088 Wembley Park to Kingsbury, Contract No. LU1742, Part 1 Text Field Records and Laboratory Testing, Report No. 7920/01/1, Rev 4 (February 1995);

5) Howard Humphries Consulting Engineers, London Underground Ltd., Earth Structures 94/95 Package A, Wembley Park to Kingsbury Ground Investigation, Interpretive Report, Report No. 84.380.1, 3151/IRWP_K.A01, Rev A01 (March 1995);

6) London Underground – Capacity Optimisation Programme, Earth Structures Assessment Programme, Geotechnical Interpretive Report, Kingsbury to Wembley Park, B088/EM4, Document No: TLL-B088-P855-EST-RPT-00001, (September 2014);

7) BGS Sheet 256 North London, 1999 (scale 1:50,000) Solid and Drift Edition, British Geological Survey;

8) Emergency Preparedness Plan (EPP), Embankment B088/EM4, Kingsbury to Wembley Park, Jubilee Line, LUL COP PDP, October 2014. Doc No: TLL-B087-P855-EST-PLN-00001;

9) Environment Agency, Operation and Maintenance Manual, Wealdstone Brook Flood Alleviation Scheme, Olympic Way to the London Underground Jubilee Line Railway Bridge at Uxendon Hill, Contract BD2, Site Reference: 38WE0101, File Reference: WN/NOMS/71, Issue - Final, Rev 0, June 2001, Doc No: D2592;

10) Tube Lines Limited, Infrastructure Portfolio, Bridges and Structures Assessment Programme, Jubilee Line, Kingsbury to Wembley Park, Structure No: W518, May 2006, Doc No: WL9342/W518/Rep/Rev0;

11) Jenkins and Potter Consulting Engineers, Structural Survey and Safe Load Assessment, Structure No. S3 : Culvert, Jubilee Line, December 1993, Doc No: 20007/DJS;



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Item No

Item Description

12) London Underground Limited, Assessment of Underbridges, Inspection and Assessment Report, Bridge No S2, Jubilee Line, Between Wembley Park and Kingsbury, February 1996,



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Appendix A – Asset Location Plans

Embankment B088/EM4

Proposed access onto embankment



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Embankment B088/EM4

Continues onto next page….

Wealdstone Brook culvert and

associated structures



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Appendix B – Conceptual Drawing of Preferred Permanent Works

LU Boundary Fence

Lineside Service

Southbound Jubilee Line Track

Notes

This drawing is not to scale. Ground profile is based on

topographical survey undertaken by LU COP Survey Team (2014)

Discrete Bored Pile (Location to be confirmed

detailed design stage)

(Regrade angle to be confirmed






1

1

?

1

1?

Proposed New Toe Drainage (Location and depth to be

confirmed at detailed design stage)

? 1

1

(Depth to be confirmed detailed design stage)

Proposed Crest Path (1000mm wide/150mm thick Type 1)

Compacted Class 1A Granular Fill, Benched into Existing Ground

Profile

Indicative Benching Only

Wealdstone Brook Culvert

W503 (Sheet Piled Wall with Concrete

Capping Beam)

W502 (Concrete Open Culvert)

B088 EM4 CDS REV3 - WhatDoTheyKnow

Documents

Transcript of B088 EM4 CDS REV3 - WhatDoTheyKnow